Starch is a representative biomass and can be a major raw material of the environmentally friendly, green industry which will be developed in the future. In particular, starch is a material which can be used in the plastics industry using petroleum resources mainly and thus can actively respond to the carbon dioxide problem now seen as the main cause of global warming. However, in case of using starch as itself in the plastics industry, it is difficult to overcome the limitation in starch's properties.
Anhydrosugar alcohol can be produced by using hexitol derived from starch and has wide applicability in the fields of drug industry, chemical industry, etc. Anhydrosugar alcohol derivatives are useful for heart and blood vessel diseases and can be used in medicaments such as patch adhesive, mouthwash, etc., and also can be applied in cosmetic compositions. In addition, if polyester, polyurethane, epoxy resin, etc. are prepared by using anhydrosugar alcohol, it is possible to provide various properties to the resin. Anhydrosugar alcohol can also be used as a raw material of plasticizer, organic solvent, etc. As such, if anhydrosugar alcohol is used in the field of resin production, heat-resistant PET, polyester fiber, high-strength sheet, film, polyurethane, etc. can be produced in a more environmentally friendly manner.
Technologies using hexitol to produce anhydrosugar alcohol have been introduced in many patents, and the production methods can be classified largely into the batch process (Starch/Starke, vol. 38, pp. 26-30, and U.S. Pat. Nos. 3,454,603, 4,564,692, 4,506,086, etc.) and the continuous process (WO 00/14081).
In a general batch process, hexitol is dehydrated by using an acid catalyst (e.g., inorganic acid, cationic resin, zeolite, etc.) in a batch reactor under reduced pressure conditions, and the reaction product is subjected to one or a combination of two or more purification processes such as distillation, recrystallization (e.g., using acetone, alcohol, ethyl acetate, water, etc.), melt-crystallization, active carbon purification, ion purification, etc., to produce anhydrosugar alcohol. On the other hand, in a continuous process—for example, as disclosed in WO 00/14081—anhydrosugar alcohol generated during the reaction is continuously extracted and isolated, and the organic solvent is recirculated, to produce anhydrosugar alcohol continuously.
In order to produce anhydrosugar alcohol economically, it is essential to employ a technology of distilling anhydrosugar alcohol from the resulting liquid of conversion reaction within a short time with high yield and high purity.
As a distillation technology of distilling the conversion liquid after dehydration reaction, batch distillation or simple distillation—wherein anhydrosugar alcohol is simply distilled under reduced pressure directly after the conversion reaction in the reactor—is known.
By the batch distillation or simple distillation, however, commercial-scale economical production is difficult since the distillation time is long. In addition, if the resulting liquid of a conversion reaction is distilled at a low temperature (e.g., 170° C. or lower), the distillation time increases, and if distilled at a relatively high temperature (e.g., 170° C. or higher), the distillation time decreases but the anhydrosugar alcohol is thermally decomposed at 170° C. or higher, and byproducts such as formic acid, furfural, etc. are generated, by which the purity of the product and the pH of the distillate are lowered. That is, as compared with the wiped-film evaporation explained below, since the batch distillation or simple distillation requires relatively longer retention time of distillate and higher distillation temperature, thermal decomposition of alcohol is induced, generating the problem of lowering purity and yield of the distillate. To prevent such a thermal decomposition, use of an additive is required.
In order to overcome the deficiencies of batch distillation or simple distillation in distilling anhydrosugar alcohol from the resulting liquid of a conversion reaction, U.S. Pat. No. 7,439,352 suggested a technology of distilling anhydrosugar alcohol by wiped-film evaporation using an external condenser. In the wiped-film evaporation technology disclosed in this US patent, the condenser is operated outside of the distillator. In this type, however, the maximum vacuum circumstance that can be formed in the distillator technically is 1 mmHg; under such a vacuum degree the distillation temperature should be 170° C. or higher in order to conduct the distillation effectively. However, as stated earlier the anhydrosugar alcohol such as isosorbide is thermally decomposed at a distillation temperature of 170° C. or higher, and thereby the distillation yield and distillation purity are lowered. Accordingly, in the above US patent, the purity of the single-step distillation product is 97.1% or the like and the distillation yield is 80% or the like. However, such levels of purity and yield are still not suitable for a commercial-scale mass-production process.
Therefore, a technology of producing anhydrosugar alcohol, which can provide high purity and high yield suitable for a mass-production process on a commercial scale even by a single-step treatment of the resulting liquid of a conversion reaction, is required.